1. Technical Field
This invention relates to a fixing belt substrate for an image forming apparatus employed in a fixing device included in an image forming apparatus, such as a copier, a printer, a facsimile machine, etc., that adopts electro-photography to fix an unfixed toner image, and to a fixing belt with such a substrate, a fixing device with the fixing belt, and an image forming apparatus with the fixing device.
2. Related Art
In response to recent demand for shortening a warm-up time and saving energy in the image forming apparatus, the fixing device that is a component of the image forming apparatus is increasingly downsized while reducing its heat capacity. At the same time, there is also increasing demand for high-speed printing. Accordingly, a fixing belt, which is a component of the fixing device to effectively convey heat to toner, is expected to be both compact and durable.
As shown in FIG. 10, a conventional fixing device 60 employs a fixing belt 61 wound around a fixing roller 62 and a heating roller 65 that acts as a winding roller wound by the fixing belt 61. A pressing roller 63 is biased toward the fixing roller 62 across the fixing belt 61. With such a configuration, a transfer medium (e.g. a transfer sheet) 64 is conveyed between the fixing belt 61 and the pressing roller 63 to fix a toner image formed on the transfer medium 64 thereonto.
Inside the hollow heating roller 65, a heater 66 such as a halogen lamp, etc., is placed along a rotation axis of the heating roller 65. Thus, radiant heat emitted from the heater 66 is transferred to the fixing belt 61 through the heating roller 65 heated by the radiant heat.
In the conventional fixing device 60, an endless belt made of polyimide resin, etc., is used as the fixing belt 61. However, in the conventional fixing device 60, since there is a relatively long distance between the nip to fix the toner image and the heat source, heat-transferring efficiency is relatively low. In addition, since the conventional fixing device 60 includes various components, and accordingly heat capacity of the fixing device as a whole is relatively large, it takes a relatively long time to reach a sufficient level needed to fuse the toner onto the recording medium (i.e., a start-up time).
Accordingly, in recent years, a fixing device as shown in FIG. 2, in which a fixing belt is driven and rotated by a pressing roller to shorten the startup time (sometimes called a quick start-up (QSU) fixing device) has been proposed. With such a structure, the problems of low heat transfer efficiency and lengthy startup time are solved by effectively shortening a diameter of it and accordingly a length of the fixing belt while laying out the heat source inside the fixing belt. To transfer heat from the heater to the nip, whereas the fixing belt 61 used in the conventional fixing device of FIG. 10 is driven by a gear train, not shown, and multiple rollers 62 and 65, by contrast the fixing belt 5 used in the fixing device of FIG. 2 is driven by the pressing roller 14 in contact with an outer circumferential surface of the fixing belt 5 as the pressing roller 14 rotates. Consequently, a larger load is imposed on the fixing belt 5 of the fixing device of FIG. 2 than in the conventional fixing device of FIG. 10, and strength thereof is possibly insufficient if it is made of resin such as polyimide, etc. For this reason, in the fixing device of FIG. 2, a fixing belt substrate 71 is frequently made of metal, such as stainless steel (i.e., SUS (Steel Use Stainless)), nickel, aluminum, copper, etc., having great strength.
Although a configuration like that described above is generally effective for its intended purpose of providing good durability, a problem arises when the fixing device of FIG. 2 is used for high-speed printing in that separation of the performance to separate a transfer medium from the fixing belt 5 is insufficient and needs to be improved. In such a situation, when the fixing belt 5 is both shortened and rotated at high speed, sufficient time cannot be ensured for toner T that has adhered to a surface of the fixing belt 5 from the transfer medium 64 due to the heat to separate from the surface of the fixing belt 5. Consequently, the transfer medium is attracted to the surface and caught by the fixing belt 5 as it rotates.
As a general solution for this problem, release of the transfer medium from the fixing belt 5 is improved by reducing a radius of curvature of the fixing belt 5 near a nip exit 7 of a sheet path while relying on rigidity of the transfer medium. However, due to the small radius of curvature, the load on the fixing belt 5 further grows as a result.
It is noted that nickel is more preferably used as the fixing belt substrate 71 of the fixing device of FIG. 2 than stainless steel because nickel has superior durability and strength, and facilitate an electroforming process manufacturing the endless belt.
When preparing the fixing belt substrate made of such electroformed nickel, prescribed amounts of phosphorus, sulfur, and carbon are mixed into the substrate to improve its heat resistance and mold releasability. Also, to enhance durability of the substrate of electroformed nickel, its crystal orientation ratio is controlled.
To ensure the mold releasability of the fixing belt to separate from toner, a releasing layer composed of fluoropolymer resin having excellent releasability is generally established on a surface of the substrate. To form the mold-releasing layer, heating to more than 300 degree Celsius is frequently required. However, subjecting the fixing belt substrate made of the electroformed nickel to such high-temperature heating makes the fixing belt substrate brittle, thereby reducing durability thereof. Moreover, reliable durability of the substrate may be hard to obtain as the fixing belt becomes more compact and rotated at higher speeds.